Apparatus of this kind has long been used for the heat treatment of different food products. The heating has been carried out directly by steam injection into the products or the vessel or by indirect heat transfer by means of agitation devices or jackets around the vessel. In order to obtain an even heat treatment the products have been subjected to a relatively vigorous stirring. The cooling has taken place by means of agitating devices or cooling jackets. In these heating apparatuses the products have been heat treated in the desired degree, cooked for a certain time, pasteurized or sterilized. The apparatuses are well suited for all kinds of products, where a certain destruction of the food material is desired, as for example soups, sauces and stews. If it is desirable that the particulate material maintain its structure, this kind of device is not suitable.
In certain connections scraped heat exchangers have been used for heat treating material in particulate form, but even if the holding time in the heat exchangers has been kept relatively short, it is not possible to completely avoid mechanical damages to the food material.
The desire to be able to heat treat particulate food material without affecting the material too strongly has increased in recent times. In the patent literature two different solutions of the problem have recently been presented.
In EP 272 087 there is described a process where a food product is transported through a rotating holding tube while simultaneously heating it until sterilization is obtained. In EP 271 915 there is described a sterilizing method with many similarities to the present invention. According to EP 271 915 the food product, which is in the shape of pieces or slices, is made to pass through a heating arrangement on a conveying belt. A first chamber in the heating arrangement through which the products are conveyed is evacuated to an absolute pressure of 50 to 150 mbar. The products are thereafter conveyed to the next chamber, in which they are heated having steam with a temperature of 125.degree.-160.degree. C. during 40 to 360 seconds. From this chamber the products are conveyed to a third chamber, where the products are cooled by means of cold sterile air.